This invention relates to multi-wavelength optical signal amplification devices and optical telecommunications systems utilizing such devices, and in particular to a dual amplification path (multichannel) optical amplifier having selectively located spectral filtering for suppressing crosstalk induced multipath interference (MPI), return loss (RL), and self oscillation, and which also provides for a desired level of noise figure performance and output power performance (i.e., pump utilization efficiency) notwithstanding increased filter insertion loss.
The term xe2x80x9ccrosstalkxe2x80x9d as used hereinafter will refer to an amplifier gain and reflection dependent phenomenon that is the genesis of multipath interference (MPI), return loss (RL), and self-oscillation, each of which are detrimental to good performance of a fiber optical communications system including an optical amplifier. The source of crosstalk induced MPI is illustrated in FIG. 1, which very generically shows a bi-directional optical amplifier including a west-to-east optical signal transmission/amplification path 1-2-3 (where 1 and 3 are a west and east reflection, respectively, and 2 is a gain from west-to-east) for wavelengths xcex94xcex1, for example, and an east-to-west optical signal transmission amplification path 3-4-1 (where 4 is a gain from east-to-west) for wavelengths xcex94xcex2. A xcex94xcex1 MPI loop is represented by nodes 1-2-3-4-1 (i.e., xcex94xcex1 input G1-RE-G2-RW). Interference between originally transmitted xcex94xcex1 signals and xcex94xcex1 signals traversing the MPI loop gives rise to MPI. Likewise, node path 3-4-1-2-3 represents a xcex94xcex2 MPI loop.
Return loss (RL) refers to xcex94xcex1 signals traversing nodal path 2-3-4 (i.e., G1,-RE-G2), and/or xcex94xcex2 signals traversing path 4-1-2, and represents the effective return reflectivity of the amplifier as seen by the communication system.
Self oscillation in the amplifier (laser oscillation) will occur when a loop or cavity is set up in which the gain exceeds the losses. Therefore, e.g., if G1+RW+G2+RE greater than 0, then lasing will likely occur. Although the following specification will describe the invention in terms of a bidirectional (two counter-directional amplification paths) optical amplifier, the invention equally applies to a multichannel, unidirectional (two co-directional amplification paths) optical amplifier.
A bi-directional optical signal amplifying device may typically provide a signal amplification path in substantially one direction (e.g., east to west) for one or more in-band communication channels within a particular frequency band (e.g., the xe2x80x9credxe2x80x9d band or hereinafter, xcex94xcex1), and a second signal amplifying path in a counter propagating direction (i.e., west to east) for one or more in-band communication channels in a different frequency band (e.g., xe2x80x9cbluexe2x80x9d band or hereinafter, xcex94xcex2). Optical amplifiers used in optical communication transmission systems typically incorporate an optical isolator in the amplification path for filtering unwanted reflections or to suppress the build up of spontaneous emission, the effects of which impair amplifier and system performance. Although it is well known that most xe2x80x9call opticalxe2x80x9d amplifiers, such as erbium doped fiber amplifiers (EDFA""s) and semiconductor amplifiers, for example, will amplify an input signal regardless of the direction that the signal enters the device, the use of an isolator in the amplification path essentially restricts such a device to substantially uni-directional operation. Optical amplifiers that are functionally bi-directional, on the other hand, and particularly those that include a substantially uni-directional amplification path for each counter propagating signal band, respectively, require means for primary signal routing through the respective counter directional amplification paths. The means for routing the primary counter propagating signals may include, for example, optical circulators or wavelength selective directional filters at each input/output port of the bidirectional amplifier. Optical circulators are not preferred primary signal routing components for use at the input/output of a bi-directional optical amplifier because they are not wavelength selective devices and they are expensive.
Currently available wavelength selective directional filters, particularly single stage components, lack the capability to provide the desired degree of spectral band discrimination within a desired narrow spectral range. For instance, in an EDFA, the gain spectrum window is on the order of 30 nm (1530-1560 nm). As shown in FIG. 1, a typical interference filter can provide approximately 10 dB spectral band discrimination through attenuation from reflection. This occurs, however, over a finite spectral range accompanied by a spectral xe2x80x9cdead zonexe2x80x9d of about 3-10 nm adjacent the signal band, instead of ideally as a step function, as shown in FIG. 5.
The dead zone thus reduces the communication signal channel availability in an already limited spectral window. Moreover, the 10 dB attenuation typically is not sufficient to eliminate MPI, RL and self oscillation effects due to, for example, reflected and double reflected xcex94xcex2 light (from connectors or Rayleigh scattering) propagating in and being amplified by the primary amplification path for xcex94xcex1 light, and vice-versa. More specifically, we have found that the most critical need for wavelength selective isolation in a bidirectional optical amplifying device is to suppress crosstalk induced MPI, RL, and self oscillation. Even with the use of isolators in the unidirectional amplification paths, MPI, for example, can occur due to light (e.g., in-band xcex94xcex1) that propagates through the amplifier, gets reflected by some mechanism in an optical path of the system, and counter propagates through the amplifier along the primary amplification path for xcex94xcex2 light (i.e., as out-of-band xcex94xcex1, by going through the 10 dB wavelength selective routing filter via the nominally suppressed path for xcex94xcex1, hitting another system reflection on the other side of the amplifier, and finally propagating again in the original intended direction of xcex94xcex1, (as in-band xcex94xcex1), to be re-amplified. One proposed solution to this problem is to increase the spectral isolation at the amplifier input/output routing locations to the primary, substantially unidirectional amplification paths for the respective in-band signals by, for example, using multi-stage filters at the input/output ports of the amplifier. This, however, also introduces increased insertion loss into the device which is not preferable as it is well known to those skilled in the art that increasing the insertion loss at the input end of an optical amplifier results in an overall increase in noise figure (due to signal spontaneous beat noise) of the device, while increasing the insertion loss at the output end of the amplifier results in decreased output power for a given pump power.
The inventors have therefore recognized a need for providing means for efficiently routing the respective communication signal bands into and out of the amplifier and, moreover, for suppressing unwanted (out-of-band) wavelength propagation through the amplifier that, if unsuppressed, results in crosstalk induced MPI, RL, and self oscillation, while not adversely impacting the noise figure and output power performance of the amplifier due to the increased insertion loss resulting from the spectral filtering.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the apparatus and method particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, an embodiment of the invention describes an optical signal amplification device having a substantially unidirectional first primary amplification path for amplifying an in-band wavelength band, xcex94xcex1, and a second substantially unidirectional primary amplification path for amplifying a different in-band wavelength band, xcex94xcex2, wherein at least one of the first and second amplification paths includes a wavelength selective insertion loss for substantially blocking the propagation of out-of-band signals in the amplification path, further wherein the location of the wavelength selective insertion loss in the amplification path is selected to provide either a target noise figure performance or output power performance from the device.
In an aspect of this embodiment, the wavelength selective insertion loss is a dielectric optical interference filter. In alternative aspects of this embodiment, the wavelength selective insertion loss may be obtained from, for example, a fiber distributed Bragg reflector, a long period grating coupler filter, and a wavelength dependent fiber coupler device such as a twisted evanescent or multiclad type WDM device.
In an aspect of the embodiment, the amplification path includes a rare earth doped optical fiber waveguide, such as an erbium doped fiber (EDF), for example. The invention, however, is not limited to fiber gain media, but may include a planar gain medium; and said gain media may include any of a variety of host glass compositions including silica, ZBLA(X), and oxyhalide (e.g., oxyfluoride) or glass ceramic compositions having appropriate lasing dopants incorporated therein.
In another embodiment, the invention describes a bidirectional optical signal amplification device that includes an input/output port for an optical signal band xcex94xcex1 and for an optical signal band xcex94xcex2, respectively, and another input/output port for an optical signal band xcex94xcex2 and for an optical signal band xcex94xcex1, respectively; a substantially unidirectional first primary amplification path for in-band xcex94xcex1 including a waveguiding gain medium and a first wavelength selective insertion loss located along said gain medium for substantially blocking propagation of out-of-band xcex94xcex2 along said first amplification path while substantially allowing propagation of in-band xcex94xcex1 along said amplification path; a substantially unidirectional second primary amplification path for in-band xcex94xcex2 including a waveguiding gain medium and a wavelength selective insertion loss located along said gain medium for substantially blocking the propagation of out-of-band xcex94xcex1, along said second amplifying path while substantially allowing the propagation of in-band xcex94xcex2 along said amplification path, first communication signals routing means coupled to the xcex94xcex1 -input/xcex94xcex2 -output port and further coupled to said first amplification path for substantially directing said xcex94xcex1 communication signals input to said port to said first amplification path; and second communication signals routing means coupled to said xcex94xcex2 -input/xcex94xcex1-output port and further coupled to said second amplification path for substantially directing said xcex94xcex2 communication signals input to said port to said second amplification path. In an aspect of this embodiment, both the directional routing means and the wavelength selective insertion loss components can include, for example, a fiber distributed Bragg reflector, a long period grating coupler filter, and a wavelength dependent fiber coupler device such as a twisted evanescerf or multiclad type WDM device.
In another embodiment the invention describes an optical signal transmission system including a transmitter and a receiver and an optical amplifier having at least a first and a second substantially uni-directional primary amplification path for different in-band wavelength bands, xcex94xcex1 and xcex94xcex2, respectively, wherein each said amplification path includes a waveguiding gain medium and a wavelength selective insertion loss located along said respective gain media for substantially preventing the propagation of an out-of-band optical communications signal along said respective amplification path, further wherein said wavelength selective insertion losses are located in their respective amplification paths to provide either a target noise figure performance or output power performance from the device.
Another embodiment of the invention describes a method for suppressing crosstalk induced MR, RL, and self oscillation in an optical amplifying device while maintaining a desired or target level of noise figure performance and output power performance of the device, including the steps of routing a communication signal substantially including an in-band wavelength band xcex94xcex1, to a first substantially unidirectional gain path; providing a wavelength selective insertion loss in said gain path for substantially blocking the propagation of an out-of-band communication signals substantially including a wavelength band xcex94xcex2 while substantially allowing the propagation of in-band xcex94xcex1; routing a communication signal substantially including the in-band wavelength band xcex94xcex2 to a second substantially uni-directional gain path; providing a wavelength selective insertion loss in said second gain path for substantially blocking the propagation of out-of-band xcex94xcex1 while substantially allowing the propagation of xcex94xcex2, wherein said insertion losses are located in said respective gain paths to provide a pre-insertion loss gain and a post-insertion loss gain that provide either a desired or target noise figure performance and output power performance from the device.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention.
Accordingly, the invention is directed to an apparatus and a method providing such features.